1. Technical Field
This invention relates generally to instruments for measuring characteristics of the human eye, and, more specifically, to hand-held devices for measuring the peripheral field of vision of an eye or any limitations to the central field of vision. The device is particularly useful for self-monitoring of persons with glaucoma or those experiencing macular degeneration.
2. Background
Glaucoma is a disease of aqueous dynamics within a closed structure, the eye. All compartments within the eye share the same watery substances, although it takes a different character in the back portion of the eye. This gel substance is called vitreous. The anterior chamber is filled with a clear, water-like fluid called aqueous. Both of these materials originate at the ciliary body. The ciliary body removes water from blood vessels and pumps the water into the eye. The rate and pressure within the normal eye is dependent on many variables but is a homeostatic system, that is, self-regulating. The normal intraocular pressure in adult humans has a range of approximately 12 to 19 mmHg.
In times of stress or hypertension, the normal eye can have an elevated pressure due to the arterial pressure increase. When dehydrated, the pressure within the eye can be significantly reduced. When the pressure inside the eye is elevated it is due to one of two factors or a combination of the two. These factors are (1) increased secretion of the ciliary body, that is, an increased amount of fluid being pumped into the eye, or (2) blockage or damage to the trabecular meshwork which is the outflow system of the eye. In some cases both of these factors are operational.
When the pressure within the eye is elevated and maintained at an elevated state, the pressure is directed backward into the optic nerve which is surrounded by a very vascular region called the cribrosa. This area is the weakest point in the wall of the eye so pressure is able to decrease the flow in the blood vascular system that supplies the optic nerve. This results in hypoxia and ultimately in the death of neurons. The most peripheral neurons suffer the greatest affects and as a result the peripheral vision becomes reduced. Normal peripheral vision has a sphere from 180.degree. to 140.degree., depending on the anatomical variations of the eye.
Patients with glaucoma are followed to determine the advance of the disease by field studies that show the extent of peripheral vision loss. Normally these patients are seen at intervals of six months or one year. It would be a great advantage for the managing physician to be able to analyze this field in the patient every day.
Macular degeneration is a leading cause of visual diminution in the elderly. Contrasted to the peripheral vision loss which occurs with glaucoma, macular degeneration leads to a painless loss of central visual acuity. The macula, or macular region, of the eye is a small, yellowish area lying slightly lateral the center of the retina. It constitutes the region of maximum visual acuity. Persons experiencing macular degeneration have a "black spot" in the center of their field of vision. As the degeneration progresses, the black spot enlarges. Progression of the disease is monitored by keeping track of the central visual field limitation. As with glaucoma, it would be a great advantage for the managing physician to be able to monitor the central vision loss on a daily basis.
It is thus the object of this invention to provide a device that allows the glaucoma patient to self-monitor peripheral vision loss for reporting to the managing physician.
It is an additional object of the invention to provide a device that allows for the measurement of central visual field limitations in persons with macular degeneration.
It is a further object of the invention that the device(s) be a hand-held unit capable of easy, at home operation.
There exist prior devices, including hand-held devices, designed to quantify refractive errors of the eye and measure visual acuity, such as those disclosed in U.S. Pat. Nos. 268,016; 484,055; 1,510,114; 1,747,844; 2,523,007; 3,664,631; 3,905,688; 4,679,921; and 5,455,645. None of the references, however, contemplates the self-measurement and reporting of peripheral vision loss by a glaucoma patient or central visual field limitations by those with macular degeneration.